This invention relates to quadraphonic audio systems, and more particularly to systems for encoding four or more individual channels of audio information for broadcast over FM multiplex radio, and for decoding and reproducing the broadcast information either as four discrete audio output signals having the directionality of the original input signals of "semi-discretely" in the manner of matrix systems, the broadcast information being fully compatible with existing monophonic and stereophonic receivers. The subject matter of this application is related to the subject matter in commonly assigned U.S. Pat. Nos. 3,937,896 entitled "Compatible Four Channel Radio Broadcast and Receiving System" and 3,940,559 entitled "Compatible Four Channel Recording and Reproducing System".
The nature of the encoding employed in the "SQ" quadraphonic record system described in applicant's U.S. Pat. No. 3,890,466, has made it possible to broadcast quadraphonic information over FM multiplex radio in a manner which is fully compatible with monophonic and stereophonic receivers. In a broadcast mode, the two composite signals produced by and SQ encoder or transduced from an SQ record, each of which contains three of the four input signals with preselected amplitude and phase relationships, can be transmitted in the same manner as a conventional stereophonic signal pair. Upon reception, a conventional monophonic or stereophonic FM receiver gives the listener a totally satisfactory monophonic or stereophonic presentation, as the case may be. If an SQ decoder of the type described in applicant's U.S. Pat. No. 3,835,255, is used in combination with the stereophonic FM receiver, the listener can obtain four-channel reproduction of the four signals contained in the transmitted composite signals. Such a system of quadraphonic transmission is at times designated a "4-2-4" system because four signals are transmitted and received via 2-channels.
It is a primary object of the invention to provide an improved system for broadcasting and receiving four independent channels discretely or semi-discretely, in a manner that is fully compatible with existing SQ decoders, as well as with existing stereophonic and monophonic receivers.
As further background for understanding the present invention and the differences between it and the more simplified method of SQ broadcasting alluded to above, it will be useful to discuss in further detail the characteristics and limitations of simply transmitting the two composite encoded signals. Employing the SQ system, the broadcaster can use an SQ encoder, of which several types are disclosed in applicant's U.S. Pat. No. 3,890,466 for combining the four channels of a four-channel program, either live or taped, into two composite signals, and transmitting them through existing FM stereo transmitter equipment. This approach provides satisfactory results at the receiver as long as there are no soloists or important instruments placed in the center-back of the quadraphonic field. It is a characteristic of the basic SQ code that center-back signals are encoded in phase opposition and thus are not received through a monophonic receiver; however, they are reproduced normally with stereophonic FM receivers and are properly decoded when replayed through an SQ encoder. Thus, the SQ code does not significantly handicap a record producer because he can be careful to avoid placing soloists in the "dead back" of the audience.
Unfortunately, the broadcasting program director often does not have the opportunity to carefully plan and edit the material being encoded for broadcast. He may have to transmit any four-channel program through the SQ system, for example, from an existing tape, which might contain a center-back soloist, or a live rock concert during the broadcast of which an artist might wander between the back-channel microphones. To accommodate these contingencies, the broadcast producer can use the "forward-looking" encoder illustrated in FIG. 20 of the aforementioned U.S. Pat. No. 3,890,466 which treats the center-back signal in an in-phase manner as if it were a center-front signal. Using this encoder, the sounds around the front and sides of the quadraphonic field, including left-back, left-front, center-front, right-front, and right-back channels are ideally coded, the signals between the back channels are redistributed among all the channels, and the center-back signal is moved forward to the center-front position. With this code, then, all listeners, monophonic, stereophonic and quadraphonic, hear all of the signals, but the quadraphonic listener perceives center-back signals as if they originated at center-front. In any event, it will be appreciated that the quadraphonic listener, using a matrix decoder, does not hear four discrete signals; each is "contaminated" to some degree with lower level signals from two other channels, as is inherent in "matrix" four-channel systems.
Although the above-described SQ system of broadcasting provides generally acceptable quadraphonic reproduction at the receiver, which can be further improved to approach the quality of discrete reproduction through the use of logic and control circuitry in conjunction with the matrix decoder, there is considerable current interest in providing a system for broadcasting four channels in a manner which allows fully discrete presentation at the receiver. Again, such a system should still retain full compatibility with conventional stereophonic and monophonic receivers and be capable of operation on frequency channels allocated by the Federal Communications Commission. To achieve these ends, one can first appropriately combine the four signals of the four-channel program into a two-channel mono-compatible stereophonic program for transmission over existing FM broadcast channels and also provide for transmission of two additional sets of signals which, upon reception followed by appropriate combination of received signals, will enable restoration of the original four program signals. In one proposed system of which applicant is aware, four audio signals are combined or matrixed to form a pair of signals designated (LF+LB) and (RF+LB) and the sum and difference of these signals are transmitted by an FM multiplex transmitter. The four program signals are also matrixed into two additional signals (LF+RF)-(LB+RB) and (LF+RB)-(RF+LB) which are transmitted simultaneously as modulation on additional subcarriers. Upon reception, the four sets of matrixed signals are appropriately recombined to derive the original four audio signals LF, RF, LB and RB. Such a system is generically designated as a 4-4-4 system because the four original program signals are combined and transmitted as four matrixed signals and upon reception are recombined back to the four original program signals. Also it has been proposed, as a means of saving spectrum bandwidth, that only the first three of the four matrixed signals be transmitted. This is known as a "4-3-4" or semi-discrete approach, resulting in reconstitution of program signals which are "contaminated" with signals from other channels to a lesser degree than that which occurs with a 4-2-4 system. A number of systems employing this basic scheme of signal combination, including the system described in Dorren U.S. Pat. No. 3,708,623 have been evaluated by the NQRC (National Quadraphonic Radio Committee). It is unnecessary for present purposes to discuss the merits of the above described sets of signals, except to point out that each of them statistically carries equal amounts of power with the consequence that it is necessary to transmit a relatively high total signal energy to perform the "discretizing" operation.
While the advantages to be derived from "discrete" or semi-discrete quadraphonic broadcasting are at this time inconclusive, and a variety of proposals are being considered, it would be desirable, in the event that a discrete system of quadraphonic broadcasting is adopted, that such systems be compatible with SQ decoders and decoder-equipped radio receivers currently in the hands of consumers. It is an object of the present invention to provide a four channel system of broadcasting which allows fully discrete or semi-discrete reception but which is also compatible with existing monophonic, stereophonic and SQ receiving and reproducing equipment.
Furthermore, it should be noted that with the systems described in the previous paragraph the monophonic listener hears the sum of the signals (LF+LB+RF+RB) while the stereophonic listener hears the (LB+LF) signal in the left channel and the (RF+RB) signal in the right channel; and it has been demonstrated through tests conducted by the Federal Communications Commission and reported in FCC's Project Number 2710-1 dated August, 1977 that SQ-coded signals in either the monophonic or the stereophonic mode are preferred by the listeners to the signal display as defined by the equations given immediately above. Thus, another object of this invention is to provide to the stereophonic and monophonic listeners the benefit of superior SQ sound at the same time offering an improved means of transmitting a program in either the semi-discrete or fully discrete transmission modes.